The present invention relates to an apparatus and method for optimizing a compressed air system used to apply liquid coatings while producing a high quality film coating.
Industries that apply liquid coatings cover a wide range of products including automotive, containers, furniture, machinery, aircraft, electronics, and shipbuilding. Approximately 1 billion gallons of liquid coatings are applied annually in the United States. About 10 percent of these liquid coatings are applied using conventional low-volume, high pressure, compressed air systems (CAS).
Compressed air spray systems have been in use for over 40 years. CAS systems use high pressure (40-70 pounds per square inch [psi]) air to atomize a liquefied stream of paint. The high energy air stream is mixed with paint, producing an atomization that is generally very fine and easily applied. Thus, the system is capable of producing a very good finish with high quality visual characteristics while easily covering a large surface area in a relatively short time span.
A disadvantage associated with CAS systems is that the high degree of atomization produces a very fine spray that is highly susceptible to overspray. Such spray characteristics result in more paint being used to compensate for overspray waste and a relatively inefficient transfer efficiency (TE) of approximately 20 to 40 percent (TE=percentage of coating gained on a sprayed part relative to the weight of coating sprayed).
Another disadvantage associated with CAS systems concerns volatile organic compounds (VOC) emissions. The consistency of most coatings is too thick for effective spray application. Thinners or solvents are introduced to decrease viscosity and thereby facilitate spray application. Examples of such thinners and solvents include petroleum spirits, mineral spirits, toluene, xylene, solvent naphtha, esters, alcohols and ketones. The high degree of atomization and relatively low transfer efficiency associated with CAS systems is conducive to relatively high levels of VOC emissions from the solvents and thinners.
Concern over VOC emissions and paint overspray has led to the development of High Volume Low Pressure (HVLP) spraying systems. An HVLP system delivers paint using a large volume of air (100 cfm) while operating at a relatively low air pressure, typically between 3 to 6 psi and not exceeding 10 psi. A CAS system typically operates between 45 to 60 psi. The low pressure high volume design of the HVLP system produces a transfer efficiency as high as 85% due mainly to the larger droplet size distributions of the spray. Large droplets and high transfer efficiency translate into reduced VOC emissions and paint consumption.
Environmental concerns over VOC emissions have led some local governments to require the use of HVLP spray systems. Southern California and the San Francisco Bay area each promulgated rules requiring the use of HVLP spray systems while applying refinish materials.
HVLP systems have several disadvantages. HVLP paint spray systems use large volumes of air and energy in comparison to CAS systems. HVLP systems also require most coatings to be greatly thinned to produce an acceptable spray. If excessive thinning is required, an operator may need to apply multiple coats to produce a desired finish. High viscosity paints such as latex may be too thick for an HVLP system. HVLP systems are also inappropriate for spraying large areas since the rate of application is relatively low in comparison to compressed air systems. Furthermore, the coating quality generated from HVLP systems, when operated at recommended settings is unsatisfactory to many users. As a result, many commercial spray coating facilities compensate by increasing the air flow rate above the recommended settings in an attempt to improve atomization and coating quality. Elevated air flow rates increase overspray, waste, and energy usage when the HVLP system is modified.
Thus, there is a continuing need for an optimized compressed air system which can achieve the high transfer efficiencies of an HVLP spray system while producing a high quality coating.